Pad mount, yes, pole mount no. I did a 1200 amp single phase service years ago, poco didn’t change the existing transformer for a 400 amp service, and just overfused it. It blew up a couple of months later, so they put a bigger can on the pole.
The only thing you will supply (without additional transformation) from the high leg is three phase loads which you apparently don't have any as of now or straight 240 volt single phase loads. If those loads are pretty limited you still end up needing oversized transformer to supply the bulk of what you have.
I ran into this issue with a old building, POCO did not want to do a 1000A single phase, and also an elevator was 240V and that was the 208 deal breaker. It already had 800A bus duct service.
Would be the same for a closed delta.
The gotchya is this:
Sorry you're correct. What I was thinking of is a four-wire delta. Three-wire delta services are a rare occurrence for me.
The drive can usually handle it, the internal surge protection however isn't set up for more than either 120 or 277 nominal to ground (yes there is 480 delta both three wire and with high leg) Whether it is a three or four wire delta it has same problem, volts to ground on at least one or more lines is too high for the surge protector. Some have the ability to disable that surge protector and then can be used though it might be good idea to add additional protection of some sort. And as mentioned open delta isn't the problem, closed delta has same voltages phase to phase and phase to ground. The problem with delta systems is loading balancing the source - open delta can not take balance three phase load without upsizing a portion of the source. Often there is a large "lighting pot" and a smaller high leg pot, when majority of load is 120/240 single phase but there is limited three phase load expected to be served. I have even seen full delta systems but with large lighting pot and two smaller pots to make up the rest of the system. Again they are expecting majority of load to be 120/240 single phase but there is some three phase load as well. Then there is times when primary voltage only has two phase conductors and the neutral- no choice but open delta system here. If actual three phase load is significant enough then they must size the "lighting pot " larger than they normally would if it were a full delta system.
Kind of covered some my answer in the above reply, every application has it's own conditions. Answer will depend on how large the "lighting transformer actually is, how large the stinger transformer actually is, what loads can operate on the high leg and what loads can not. Yes you can operate many straight 240 volt loads from high leg and one the other legs, but you must also use straight 240 volt rated breakers and not 120/240 rated breakers on those loads. An application like you mentioned is much simpler on a 208/120 wye system, though output of resistance heat items is reduced by about 25% and also needs some consideration to determine if it will be sufficient enough.
Hey Tortuga, how many times have you run into this unstable voltage across the open phases? Did you meter it and if so what kind of fluctuation were you getting? There are lots of open deltas in Seattle and I don't recall ever hearing complaints, but I am interested as I am setting up a new lab on an open Delta, and there will definitely be some potentially "sensitive electronics" at the facility.
Seems more likely to happen if the stinger pot is much smaller than the lighting pot, and the fluctuations would likely occur when there is other load changes. You have both source coils in series when connecting a load between A and B, add any significant load to a somewhat undersized BC coil and you will effect the AB voltage. AC and AN CN loads also will impact this though it is likely the AC transformer is larger than BC transformer so needs to be even larger load to have similar effect. If both transformers are same size then heavy loading of the AC side with 120 volt loads can contribute as well, though that should effect the AC voltage also, or possibly that one larger three phase load is causing some the issues when it cycles on/off.
Agree. And even if both transformers are the same size, because their secondaries are in series the source impedance presented across A-B will be twice that across A-C and B-C. And so the voltage drop for a given load current on A-B will be twice what it would be on A-C or B-C.
The problem was only on single phase 240 loads across A-B.
Are you working under the 2020 NEC? If so, are you having to deal with 230.46?
